1. Field of the Invention
The present invention relates to a pseudo-cubic function generator circuit for generating an output voltage which approximates a cubic function in response to an input voltage.
2. Description of the Related Arts
Electronic apparatuses often use quartz crystal units as a reference for the time, frequency, and the like. A representative crystal unit employs a quartz crystal blank of AT-cut. While the crystal unit having an AT-cut crystal blank has a high stability in resonance frequency, the frequency varies over several tens of ppm (parts per million) at maximum, for example, in response to a change in temperature from xe2x88x9230xc2x0 C. to +80xc2x0 C. It is known that the AT-cut crystal unit responds to the ambient temperature by exhibiting a frequency change which is represented by a cubic function curve that has an inflection point substantially in the vicinity of room temperature.
Recent electronic apparatuses are required to be highly functional and accurate. With such requirements, a quartz crystal oscillator using a crystal unit is often required to oscillate highly stable frequencies. To meet such requirements, a voltage controlled crystal oscillator is applied, for example, with a control voltage, which changes as a cubic function of the temperature, to maintain a constant frequency.
For generating a control voltage which changes as a cubic function of the temperature, Japanese Patent Laid-Open Application No. 11-68463, (JP, 11068463, A), for example, discloses a temperature compensated crystal oscillator which comprises a first circuit for converting a signal indicative of a temperature detected by a temperature sensor to a square curve signal in a higher temperature range than a normal temperature; and a second circuit for converting the detected signal to a square curve signal in a lower temperature range than the normal temperature, and combines the signals from the first and second circuits to generate a signal which approximates a cubic function.
Another technique for generating a cubic function voltage may take advantage of square law characteristics in the input/output relationship of FET (field effect transistor) to square an output detected by a temperature sensor, and multiply the resulting signal by the output detected by the temperature sensor using a multiplier circuit to generate the cubic function voltage.
In General, however, the circuit configured as described above is highly susceptible to superimposition of a noise component on the resulting signal because the circuit largely amplifies a small voltage change. The foregoing circuit is also problematic in a complicated circuit configuration, a large number of elements used in the circuit, increased power consumption, and a higher cost.
It is an object of the present invention to provide a pseudo-cubic function generator circuit which is capable of directly generating a large voltage change in a simple circuit configuration, reducing a noise component and cost, and readily setting a parameter for controlling the shape of the cubic function curve.
The object of the present invention is achieved by a pseudo-cubic function generator circuit including a CMOS (complementary metal-oxide-semiconductor) inverter which is applied with a detected voltage of a sensor and supplied with a power supply voltage, and a voltage divider circuit for dividing the power supply voltage, wherein the CMOS inverter has an output terminal connected to a voltage division point of the voltage divider circuit to generate an output voltage from the voltage division point.